The Megger

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Continuing on the geekery of electrical test instruments, Pictured here is the Megger. So named because it measures megohms.  Ohms is the unit of electrical resistance. If one volt is applied to a one ohm resistor a current of one amp will flow and one watt of heat will be generated The Megger can measure up to infinity which is effectively an open circuit.  This is achieved by generating high voltages with the hand cranked generator seen on the right of the instrument. Many practical jokes have been played using the megger to zap a colleague.  These zappings are outside the capability of the batter powered Avo

When would it be used?  Mainly for detecting earth faults.  An earth fault occurs when the insulation on cables or equipment breaks down and allows electricity to escape to earth which in Explorers case is the metal structure of the vessel.  A single earth is not in itself dangerous, but, on a DC vessel like Explorer the problem comes when positive cable and a negative cable earth at the same time.  This is what we doctors call a short circuit with attendant risk of fuses blowing or even causing a fire.

On Explorer’s switchboard are two red earth lights.  When the switchboard is live both these lights should be illuminated.  If one is dim or extinguished that would indicate a partial or total earth.

The detective work of locating the earth is achieved initially by opening circuit breakers to determine which motor or distribution board is the culprit, open the faulty circuit breaker and both light will illuminate.  When the offending circuit is located the resistance can be measured with the megger to further localise the culprit. 

What causes an earth fault?  Mechanical damage to cables and water/moisture are the primary  causes.  Explorer does currently carry a couple of earth faults due to the aging and deteriorating state of the wiring.  These circuits are isolated.

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